Accessing preferences on the interface Illuminate\Contracts\Auth\Authenticatable suggest that you code against a concrete implementation. How about adding an instanceof check?
The method setLocale() does not exist on Illuminate\Container\Container. Are you sure you never get this type here, but always one of the subclasses?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation